System and method for absorbance modulation lithography

ABSTRACT

A lithography system is disclosed that provides an array of areas of imaging electromagnetic energy that are directed toward a recording medium. The reversible contrast-enhancement material is disposed between the recording medium and the array of areas of imaging electromagnetic energy.

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/653,766 filed on Feb. 17, 2005, and is acontinuation-in-part application of U.S. Ser. No. 11/154,352 filed onJun. 16, 2005.

BACKGROUND

The present invention relates generally to lithography, and inparticular, to zone plate array lithography.

In a zone plate array lithography system, an array of diffractive lensessuch as Fresnel zone plates may be used to form an array of tightlyfocused spots on a photosensitive layer that is on top of a substrate.For example, U.S. Pat. No. 5,900,637, the disclosure of which is herebyincorporated by reference, discloses a mask-less lithography system andmethod that employs a multiplexed array of Fresnel zone plates. Thelight incident on each diffractive lens may be controlled, for example,by one pixel of a spatial light modulator. The spatial light modulatorfor use in such a system should provide a high refresh rate, be able tooperate at short wavelengths such as under 200 nm, and be able toperform gray-scaling or intensity modulation in real time.

One commercially available spatial light modulator that may satisfy theabove requirements is the grating light valve (GLV) spatial lightmodulator made by Silicon Light Machines of Sunnyvale Calif. The GLVconsists of a linear array of pixels, and each pixel consists of sixmetallic ribbons that form a diffraction grating. Alternate ribbons maybe moved by electrostatic actuation to provide either a reflectivesurface or a grating.

The lithographic resolution, however, of such a system may be limited bythe contrast of the aerial image. The image contrast is dependent on theprinted pattern. The optical performance may be quantified bycalculating the aerial image contrast of a dense grating as a functionof the half-pitch of the grating. The image contrast, K is defined as:

$\begin{matrix}{K = \frac{I_{\max} - I_{\min}}{I_{\max} + I_{\min}}} & (1)\end{matrix}$where I_(max) and I_(min) are defined as the maximum and minimumintensities of an illumination signal that may be employed to provide adesired pattern. For example, as shown in FIG. 1, a desired pattern 10that includes alternating imaged regions (as shown at 12) and non-imagedregions may be created using an illumination signal 14. Note that thepitch p of the desired grating pattern 10 corresponds to the pitch p ofthe illumination signal 14.

The intensity profile of the illumination signal 12, however, results inan imaging pattern on a photo-resist layer 16 when imaged byillumination source signals 18 (again having a pitch p). Thephoto-resist layer 16 is supported by a wafer 20, and includes markedregions in the photo-resist layer that have been exposed by the sources18. After exposure, the photo-resist layer is developed, and the markedregions are removed, leaving exposed portions 22 of the underlying wafer20. Efforts to increase resolution (e.g., decrease the pitch p),however, may result in a degradation in image contrast, due at least inpart to the intensity profile of the illumination signal 14.

In particular, the aerial images for gratings of different periods maybe simulated assuming a zone plate array lithography system of numericalaperture (NA)=0.7 and λ=400 nm. The cross-section through each gratingmay then be averaged over several line-scans, and the image contrast maybe calculated using Equation (1) above. The image contrast may beplotted as a function of k₁, where k₁ is a measure of the lithographicresolution (normalized to the wavelength and NA), and is given by:

$\begin{matrix}{k_{1} = {\frac{p}{2}\frac{N\; A}{\lambda}}} & (2)\end{matrix}$where NA is the numerical aperture and λ is the exposing wavelength. Forexample, a system of the prior art may provide that k₁=0.32, whichcorresponds to an image contrast of about 18%. As the pitch p becomessmaller, the image contrast will be negatively affected, due in pail, tothe spatial extent of each illumination source 18.

Contrast enhanced lithography may be employed in an effort to improveimage contrast. In particular, as shown in FIGS. 2A-2D, acontrast-enhancement material 24 that is spin coated on top of aphoto-resist layer 26 on a wafer 28. The contrast-enhancement material24 may, for example, be a photo-bleachable polymer, whose absorptiondecreases (i.e., becomes more transparent) with increasing exposuredose. The intensity of transmitted light may be plotted as a function oftime for an ideal contrast-enhanced system. Prior to exposure of thecontrast-enhancement material 24, the material 24 is opaque, and almostno light passes through the material 24. After sufficient exposure byillumination beams 30, the material becomes transparent and light istransmitted in areas indicated at 32. Light is let through into thephoto-resist layer only where the exposure dose is high enough to bleachthe contrast-enhancement material 24 completely. This increases thecontrast of the image recorded in the photo-resist. An antireflectivecoating between the photo-resist and the wafer may also be employed.

As shown in FIG. 2B, illumination is able to reach defined regions 34 ofthe wafer 28 only in areas where the contrast-enhancement material 24has become transparent (as shown at 32). The contrast-enhanced material24 is then removed as shown in FIG. 2C using a suitable medium in (suchas water) in which the contrast-enhancement material 24 will dissolve.The defined regions 34 are then removed through photo-resistdevelopment, leaving openings 36 in the photo-resist layer 26 throughwhich portions of the wafer 28 may become exposed as shown in FIG. 2D.

By employing a contrast-enhancement material and by controlling thephoto-bleaching rate of the contrast-enhancement material, as well asthe clearing dose of the photo-resist, one may enhance the contrast ofthe aerial image that is recorded in the photo-resist. The contrastenhancement material behaves, in essence, as a contact mask, whichincreases the contrast of the image recorded in the photo-resist. Thecontrast enhancement material is removed from the resist prior todevelopment. If the contrast-enhancement material is incompatible withthe photo-resist, a barrier layer is needed between thecontrast-enhancement material and the photo-resist. This also incurs anadditional step for removal of the barrier layer after exposure. Thereare several commercially available contrast-enhancement materials, someof which are water-soluble.

Ideally, such contrast-enhancement materials would become bleached bythe illumination signal 14 in an on/off step pattern that provides aninstantaneous step at the edge of each illumination beam. Since thebeams, however, provide an intensity profile as shown in FIG. 1B, thecontrast-enhancement material bleaches in varying amounts with distancefrom the center of each illumination beam. This limits resolution.Moreover, repeated illumination near a non-imaged area may accumulateover time, and may eventually reach a threshold within the material forbecoming transparent.

Contrast-enhancement may also be achieved by diluting the developer orby using thin photo-resist layers, but such systems may also involvedifficulties such as increased line-edge roughness, as well asdifficulties with pattern transfer respectively.

There is a need therefore, for an imaging system that more efficientlyand economically provides increased image contrast in mask-lesslithography.

SUMMARY

In accordance with an embodiment, the invention provides a lithographysystem that provides an array of areas of imaging electromagnetic energythat are directed toward a recording medium. The reversiblecontrast-enhancement material is disposed between the recording mediumand the array of areas of imaging electromagnetic energy.

In accordance with another embodiment, the invention provides alithography system that includes a first interference system forproviding an interference pattern of a first electromagnetic field of afirst wavelength on a surface of a recording medium, as well as areversible contrast-enhancement material being disposed between saidrecording medium and the first interference system.

In accordance with a further embodiment, the invention provides a methodof forming a lithographic image on a photo-resist material. The methodincludes the step of illuminating at least a first portion of areversible contrast-enhancement material and an associated portion of aphoto-resist material with a first electromagnetic energy field having afirst wavelength. The illumination of the reversiblecontrast-enhancement material causes the reversible contrast-enhancementmaterial to change from a first state to a second state in the firstportion. The method also includes the step of illuminating at least asecond portion of the reversible contrast-enhancement material that witha second electromagnetic energy field having a second wavelength causingthe reversible contrast-enhancement material to remain in said firststate in the second portion.

BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The following description may be further understood with reference tothe accompanying drawings in which:

FIG. 1A shows an illustrative diagrammatic view of a desired imagepattern;

FIG. 1B shows an illustrative diagrammatic view of an illuminationsignal for forming an image pattern as shown in FIG. 1A;

FIG. 1C shows an illustrative diagrammatic view of the formation of thedesired image pattern shown in FIG. 1A using the illumination signalshown in FIG. 1B in accordance with the prior art;

FIGS. 2A-2D show illustrative diagrammatic views of a process forforming the desired image pattern shown in FIG. 1A usingcontrast-enhanced lithography in accordance with another system of theprior art;

FIGS. 3A-3F show illustrative diagrammatic views of a process forforming the desired image pattern shown in FIG. 1A in accordance withanother system of the prior art;

FIG. 4 shows an illustrative diagrammatic view of a reversible contrastenhancement material in accordance with another embodiment of theinvention;

FIG. 5 shoes an illustrative diagrammatic exploded view of an array ofenergy sources and an array of diffractive elements for use in a systemin accordance with an embodiment of the invention;

FIG. 6 shows an illustrative diagrammatic view of a lithography systemin accordance with an embodiment of the invention;

FIG. 7 shows an illustrative diagrammatic view of a lithography systemin accordance with another embodiment of the invention;

FIG. 8 shows an illustrative diagrammatic view of a lithography systemin accordance with a further embodiment of the invention;

FIGS. 9A-9E show illustrative diagrammatic views of a system inaccordance with a further embodiment of the invention that may beemployed for forming marks having a scale smaller than the resolution ofthe imaging system;

FIG. 10 shows an illustrative diagrammatic view of a lithography systemin accordance with a further embodiment of the invention;

FIGS. 11A-11F show illustrative diagrammatic representations of alithography process in accordance with an embodiment of the invention;

FIG. 12 shows an illustrative diagrammatic view of a system inaccordance with a further embodiment of the invention; and

FIG. 13 shows an illustrative diagrammatic view of a system inaccordance with a further embodiment of the invention

The drawings are shown for illustrative purposes only and are notnecessarily to scale.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention provides an absorbance modulation lithography system inwhich a contrast-enhancement material may be used that is reversible inaccordance with an embodiment of the invention. The exposure dose due tothe background in each focused illumination beam may then be completelyeliminated from the contrast-enhancement material and therefore therecorded image. After reversal, the photo-resist will have no remainingrecording of the previous exposures. Two spots, therefore, may be placedcloser together than would otherwise have been possible usingconventional contrast-enhancement material. Such imaging systems may beparticularly suitable, for example, in dot-matrix printing systems inwhich each spot is printed independently of other spots. The smallestdistance between exposed spots may, therefore, be much smaller than thatdictated by the diffraction limit.

FIGS. 3A-3F show a lithography process in accordance with an embodimentof the invention. As shown in FIGS. 3A and 3B, a reversiblecontrast-enhancement material 40 on a photo-resist material 42 on awafer 44 is imaged with illumination beams 46, causing imaged regions 48to be formed in the photo-resist material 42. Once the illuminationbeams 46 cease, the reversible contrast-enhancement material 40 thenreverts back to being opaque as shown in FIG. 3C.

Then either the stage supporting the wafer is moved, or the illuminationbeams are moved. The composite is then imaged with illumination fields50 at different locations with respect to the wafer than the image beams46 as shown in FIG. 3D. This causes imaged regions 52 to be formed inthe photo-resist material 42. The imaged regions 52 may be very close tothe imaged regions 48 in the photo-resist material 42. When thereversible contrast-enhancement material is removed (as shown in FIG.3E), the photo-resist material 42 may be developed, leaving the desiredpattern of unexposed portions 54 in the photo-resist material 42 asshown in FIG. 3F.

A photochromic organic compound, which includes reversible dyes underphotochemical control, may be used for the reversiblecontrast-enhancement material. The recovery in this case occurspredominantly by thermal mechanism as is the case with spiropyrans,spirooxazines and chromenes. The thermally driven recovery may be aidedby photochemical processes as well. In other compounds, the recovery maybe predominantly photochemical (for, e.g., fulgides or arylethenes). Inthis case, exposure with a different wavelength (that does not affectthe underlying photo-resist), may be used to induce the recovery. Suchrecovery is also present in the saturable absorbers used in mode lockedlasers, although the contrast may not be particularly high. The recoveryis preferably spontaneous. The ideal material for the reversiblecontrast enhancement will have a high contrast between the transparentand the opaque states. It will also have very fast photobleaching andrecovery kinetics. It should also be easily spin-coated into a thin filmon top of the photoresist on a flat substrate. Finally, it should beeasily removed after exposure without affecting the underlyingphotoresist. Other potential candidates for this material are:nanoparticles dispersed in polymer matrices, photochromic dye molecules(described above) dispersed in polymer matrices, thin films of Antimony,or semiconductor saturable absorbers used in mode-locked lasers, andcarbon nanotubes. For example, FIG. 4 shows a substrate material 56(e.g., a polymer matrix) that includes photochromic dye molecules 58.

A lithography system in accordance with an embodiment of the inventionmay be used with arrays of a variety of focusing elements, such asFresnel zone plates as disclosed in U.S. Pat. Nos. 5,900,637 and6,894,292, the disclosures of which are hereby incorporated byreference. As shown in FIG. 5, an array of focusing elements 53 may bearranged on a substrate 55, wherein each zone plate defines a unit cell.The array is supported on a thin membrane with vertical,anisotropically-etched silicon (Si) joists 57 for rigid mechanicalsupport that divide rows of unit cells. In alternative embodiments ofthe invention, the joists may not be necessary, and the substrate neednot be formed of silicon. The membrane is formed of a material that istransparent to the beam source. If the source is 4.5 nm x-ray, then themembrane may be formed of a thin carbonaceous material. If deep UVradiation is used, the membrane may be made of glass, and the zoneplates may be made from phase zone plates, e.g., grooves cut into aglass plate or membrane.

An array of individually selectable sources 59 is also provided on asupport substrate 51 such that each source is aligned with one of thefocusing elements 53. The sources may be semiconductor lasers, diodelasers, light emitting diodes (LEDs), vertical cavity surface emittinglasers (VCSELs) or a variety of other sources such as x-ray sources orelectron beam sources. These may be microfabricated in arrays, and mayprovide extremely high modulation frequencies (about 1 GHz), whichtranslates to very high patterning speeds. In further embodiments, theeach source 59 may include a micro-lens and/or phase-shift mask thatprovides a de-focused pattern (e.g., a ring phase shifted to π/2) at thecorresponding focusing element 53 to narrow the point spread function atthe image plane.

The focusing elements may be any of a variety of diffractive and/orrefractive elements including those disclosed in U.S. patent applicationSer. No. 10/624,316 filed Jul. 22, 2003, (the disclosure of which ishereby incorporated by reference) which claims priority to U.S.Provisional Applications Ser. Nos. 60/397,705 and 60/404,514, including,for example, amplitude and/or phase Fresnel zone plates, blazed Fresnelzone plates, bessel zone plates, photon sieves (e.g., amplitude photonsieves, phase photon sieves, or alternating phase photon sieves), andthe diffractive focusing elements may be apodized. These may bemicrofabricated in large arrays as well, and may be designed tocompensate for wavefront characteristics in the radiation output fromthe source array to achieve, for example, the smallest possible focalspot.

FIG. 6 shows an example of a lithography system 60 in accordance with anembodiment of the invention. The system 60 includes a laser source 62,an imaging lens 64, a spatial filter 66, a collimating lens 68, aspatial light modulator 70, telescoping lenses 72, 74, a diffractiveelement array 76 including a plurality of diffractive elements 78, and aphoto-resist material 84 that is covered by a reversiblecontrast-enhancement material 82 and is supported by a stage 86. Thereversible contrast-enhancement material 82 is used as a top coat on thephoto-resist material 84. The diffractive elements may be zone plates(such as Fresnel zone plates) or photon sieves.

Illumination from the laser source 62 is directed toward the spatiallight modulator 70, and light is selectively reflected from the spatiallight modulator 70 onto specific zone plates 78 of the zone plate array76 for forming the desired imaging pattern. The illuminated zone plates78 direct focused illumination onto desired locations of thephoto-resist material 86 as shown at 80. The scanning stage may be movedto provide the desired image pattern over the entire photo-resistmaterial 84. In this example, the exposing wavelength bleaches thereversible contrast-enhancement material. When the exposing illuminationis turned off (either by the laser turning off or the spatial lightmodulator moving the illumination beam away from the zone plates 78),the reversible contrast-enhancement material relaxes spontaneously. Thetiming of the on and off states of the exposing light is controlled toachieve the appropriate exposure of the photo-resist.

In accordance with another embodiment, a system 90 (shown in FIG. 7) mayinclude a laser source 62, an imaging lens 64, a spatial filter 66, acollimating lens 68, a spatial light modulator 70, telescoping lenses72, 74, a zone plate array 76 including a plurality of zone plates 78,and a photo-resist material 84 that is covered by a reversiblecontrast-enhancement material 82′ and is supported by a stage 86 similarto those shown in FIG. 6. As also shown in FIG. 7, however, therelaxation of the reversible contrast-enhancement material 82′ of thesystem 90 may be assisted by exposure with infra-red (IR) illumination(if, for example, the relaxation is thermal), or by exposure withillumination having a second wavelength (λ₂) that is different than thewavelength (λ₁) of the laser source 62 (if, for example, the relaxationis photo-initiated).

The exposure of the IR or λ₂ illumination may be provided by a source92, shutter or acousto-optic modulator (AOM) 94, an imaging lens 96, aspatial filter 98, a collimating lens 100 and a beam splitter/combiner102. This system 90 provides that the exposure of the IR or λ₂illumination for relaxing the reversible contrast-enhancement material82′ floods the reversible contrast enhancement material 82′ after eachtime that the source 62 and modulator 70 are used for imaging a spot onthe photo-resist material 84. The AOM 94 may be used to switch theexposure for relaxation in rhythm with the exposing wavelength. Therelaxation exposure is toned on when the exposing wavelength is turnedoff.

In accordance with a further embodiment of the invention, a system 110may provide for relaxation of selective regions of the reversiblecontrast-enhancement material as shown in FIG. 8. In particular, thesystem 110 may include a laser source 62, an imaging lens 64, a spatialfilter 66, a collimating lens 68, a spatial light modulator 70,telescoping lenses 72, 74, a zone plate array 76 including a pluralityof zone plates 78, and a photo-resist material 84 that is covered by areversible contrast-enhancement material 82′ and is supported by a stage86 similar to those shown in FIG. 6. As also shown in FIG. 8, however,the relaxation of the reversible contrast-enhancement material 82′ ofthe system 110 may be assisted by exposure with IR illumination, or byexposure with illumination having a second wavelength (λ₂) that isdifferent than the wavelength (λ₁) of the laser source 62. The exposureof the IR or λ₂ illumination may be provided by a source 112, an imaginglens 114, a spatial filter 116, a collimating lens 118, a second spatiallight modulator 120, a mirror 122, and a beam splitter/combiner 124.This system 110 provides that the exposure of the IR or λ₂ illuminationfor relaxing the reversible contrast-enhancement material 82′ isselectively directed to desired locations of the reversible contrastenhancement material 82′ immediately after imaging of the associatedlocations on the photo-resist material 84. The focusing of the IR or λ₂illumination may be achieved by using the same zone plates 78 of thezone plate array 76, either by providing that the zone plates 78 aresufficiently focused at the desired locations to achieve relaxation ofthe reversible contrast-enhancement material, or by providing that thezone plates 78 are designed to focus illumination of wavelengths λ₁ andeither IR or λ₂ at the same focal distance.

In accordance with yet another embodiment, the invention may providethat a shaped-beam may be used to create a null in the reversiblecontrast-enhanced material at the desired exposed location. For exampleand as shown in FIGS. 9A-9E, a photo-resist material 130 may be topcoated with a reversible contrast-enhancement material 132 (and may alsoinclude an anti-reflective coating between the photo-resist material andthe wafer 134, as well as a barrier layer if desired). The reversiblecontrast-enhancement material in this example, however, is initiallytransparent to exposing illumination having a wavelength λ_(exp). Thereversible contrast-enhancement material becomes opaque when exposed toillumination having a transforming wavelength λ_(tr).

In particular, as shown in FIG. 9B, an annular ring spot is firstcreated using the illumination λ_(tr) as diagrammatically shown at 136.The illumination 136 causes the reversible contrast-enhancement material132 to develop opaque regions 140 while leaving transparent regions 138as shown in FIG. 9B. The transparent region 138 that is in the center ofthe ring spot is then used as a mask for illuminating the photo-resistmaterial 130 at that location 142 using exposure illumination 144(λ_(exp)) as shown in FIG. 9C. The imaged region 142 may be smaller thanthe smallest imaging element size possible using conventional imaging,and is made possible by using the reversible contrast-enhanced materialin accordance with an embodiment of the invention.

The system may then either wait until the reversiblecontrast-enhancement material relaxes and becomes transparent again, ormay apply a relaxation illumination 146 (e.g., IR or another wavelengthillumination λ_(ret)) to cause the reversible contrast-enhancementmaterial to relax and become transparent again as shown in FIGS. 9D and9E. The wavelength λ_(ret) may be equal to λ_(tr).

As shown in FIG. 10, a system 150 for providing the annular ring spotused in FIGS. 9A-9E may include the laser source 62, the imaging lens64, the spatial filter 66, the collimating lens 68, the spatial lightmodulator 70, telescoping lenses 72, 74, the diffractive element array76 including a plurality of diffractive elements 78, and thephoto-resist material 84 that is covered by the reversiblecontrast-enhancement material 82″ and is supported by the stage 86 asdiscussed above with reference to FIG. 6. The system also includes asource 152 (of λ_(tr) illumination), a shutter or acousto-opticmodulator (AOM) 154, an imaging lens 156, a spatial filter 158, acollimating lens 160, a phase plate 162, and a beam splitter/combiner164. The phase plate 162 provides a phase shift in the λ_(tr)illumination (for example a spiral phase shift) to provide the annularring spot illumination 136 shown in FIGS. 9B and 9C.

In accordance with another embodiment, the system may provide anabsorbance modulation lithography system in which a reversiblecontrast-enhancement material 174 deposited on a photo-resist 172 on asubstrate 170 as shown in FIG. 11A is illuminated with a standingillumination waveform 176 as shown in FIG. 11B. The waveform 176 isprovided at an imaging frequency of λ_(im). Another standing waveform178 may also be provided at a reversing frequency λ_(rev) at which thereversible contrast-enhancement material may be actively reversed. Thewaveforms 176 and 178 are provided 180 degrees out of phase from oneanother and may be sufficiently close in frequency that they remainsynchronous over the imaging region. The photo-resist is chosen suchthat it may be imaged by an electromagnetic field at a frequency λ_(im)but not by a field at a frequency of λ_(rev).

The standing waves may be formed by interference of monochromaticcoherent sources as discussed below with reference to FIGS. 12 and 13.As shown in FIG. 11B, both the waveforms 176 and 178 may be provided atthe same time such that the nodes of the grating at λ_(im) coincide withthe anti-nodes of the grating at λ_(rev). The intensities of theillumination at the frequencies λ_(im) and λ_(rev) may be adjustedindependent of one another to provide that the resist 172 is exposed inthe desired amount for specific applications. As shown in FIG. 11C,following exposure, the entire material 174 is reversed either by floodexposure to illumination at the wavelength λ_(rev) or by waiting for thematerial 174 to reverse on its own or due to application of thermalenergy as discussed above. The wavelength of the reversing illuminationλ_(rev) may be greater than that of the imaging illumination wavelengthλ_(im) but less than twice the imaging illumination wavelength(<2λ_(im)).

The substrate 170 may be stepped by a small amount as determined by thedesired pitch of the final grating. In further embodiments, rather thanstepping the substrate, the fringes of the incident gratings may bemoved by changing the phase of the incident illumination. These stepsmay be implemented without requiring the removal of the substrate fromthe lithography tool. As shown in FIG. 11D, the photo-resist 172 maythen again be imaged (optionally at the same time as the π/2 shiftedreversing illumination) at different areas of the photo-resist 172. Theprocess may then be repeated as necessary to provide a photo-resist 172with very finely developed features in the photo-resist 172 as shown inFIG. 11E. The reversible contrast-enhancement material 174 may then beremoved as shown in FIG. 11F.

As shown in FIG. 12, a system in accordance with an embodiment of theinvention includes in imaging laser source 180 that is directed viaimaging optics toward a reversible contrast-enhancement material 182 ona photo-resist 184 on a substrate 186. The imaging optics include a beamsplitter 190, an adjustable delay unit 192, mirrors 194 and 196. Thesystem may also include a reversing illumination laser source 200 thatdirects reversing illumination toward the surface of the material 182via a beam splitter 202, an adjustable delay unit 204, mirrors 206 and208. The optical path lengths from the laser 180 are controlled toensure that the two portions of the imaging illumination provideinterference at the material 182 (from +/− angle α) that yields astanding waveform of λ_(im) on the material 182. Similarly, the opticalpath lengths from the laser 200 are controlled to ensure that the twoportions of the imaging illumination provide interference at thematerial 182 (from +/− angle β that is less than α) to yield a standingwaveform of λ_(rev) on the material 182. Since the period is defined asP=π/sin θ, the period of each of the waveforms may be set to be equal toone another by requiring that λ_(im)/sin α=λ_(rev)/sin β.

A lithography system in accordance with a further embodiment of theinvention includes a diffraction grating 220 that receives incidentimaging and optional reversing illumination (at wavelengths λ_(im) andλ_(rev)). The grating and incidence angle are adjusted to providepositive and negative first order diffraction as shown, and the firstorder diffracted illumination is received by additional gratings 222 and224 as shown in FIG. 13. Each of the gratings 222 and 224 providespositive and negative first order diffraction as well, and the positivefirst order diffraction from one grating is interfered with the negativefirst order diffraction from the other grating as shown in FIG. 13. Theinterference of the fields occurs at a reversible contrast-enhancementmaterial 226 on a photo-resist 228 on a substrate 230. One of thegratings 224 includes a phase delay unit that comprises a substrate 232(such as glass) having a thickness (d) such that the Difference in phaseshifts is provided by the substrate 232 to both wavelengths ofillumination that pass through the substrate 232. In particular, thephase shift (φ) in the material 232 is provided by the optical pathlength through the material 232 divided by the wavelength of theillumination in the material 232. The difference in phase shiftφ_(im)−φ_(rev) is set to equal π, and the thickness d is determinedknowing λ_(im), λ_(rev), and the indices of refraction of the material232 at the two wavelengths solving for the above equations. The system,therefore, provides an achromatic interference lithography system.

Those skilled in the art will appreciate that numerous modifications andvariations may be made to the above disclosed embodiments withoutdeparting from the spirit and scope of the invention.

1. A method of forming a lithographic image in a photoresist material,said method comprising the steps of: illuminating with a firstwavelength in the form of a standing illumination waveform a region of areversible contrast-enhancement material located on top of saidphotoresist material wherein said standing illumination waveform has amultiplicity of smaller regions of minimum intensity adjacent to regionsof higher intensity; and illuminating simultaneously with a secondwavelength at least the regions of minimum intensity provided by saidfirst illumination waveform to produce an image in said photoresistmaterial.
 2. The method as claimed in claim 1, wherein the standingillumination waveform provided by said first wavelength is produced byinterfering at least two beams of said first wavelength.
 3. The methodas claimed in claim 2, wherein said step of illuminating with the secondwavelength is provided by interfering at least two beams of said secondwavelength to produce a standing illumination waveform of said secondwavelength, and wherein the spatial phase of the standing illuminationwaveforms created by said first and said second wavelength areapproximately 180 degrees out of phase.
 4. The method as claimed inclaim 3, wherein said first wavelength and second wavelength areprovided using diffraction gratings.